The recovery of gold and silver by the standard cyanide extraction method from either fresh ores or tailings from previous mining operations can be rendered uneconomical if the presence of base metals, such as copper or zinc, competes with gold and silver for the combination with the available cyanide. When these base metals are present in significant amounts, the cyanide consumption which will vary from 0.2 to 1.0 Kg per ton of ore for a normal cyanide extraction of gold will be raised to values that may be ten times higher, thus removing all profitability from the exploitation of low grade ores.
The base metals and silver in minerals associated with gold are very often bonded in one form or another to sulfur, arsenic or oxygen derivatives such as sulfides, arsenides, sulfates, sulfites, oxides, hydroxides, carbonates or mixed salts containing metal to sulfur or metal to oxygen bonds. This is particularly true with tailings which have been exposed to weathering over several years, the chemical entities associated with metals being then fairly complex and partly oxidized.
At the present time, it is not a common practice in the gold extraction industry to implement significant efforts in order to remove base metals and silver prior to cyanide extraction of gold. If a low cost method for base metals removal could be developed, such a preliminary step could prove useful by the reduction of cyanide cost of subsequent gold extraction and by the value of the reclaimed base metals and silver.